Electrical power cables typically include a conductor core and an outer jacket or sheath. The “conductor core”, as used herein and throughout the specification and claims, may be, for example, a single metal wire, multiple small wires twisted together to make a “stranded” cable, multiple insulated wires, or other types of electrical conductors acting together to serve a particular power function (e.g., three-phase connection). The term “sheath,” as used herein and throughout the specification and claims, means the outermost covering surrounding the conductor core, whether of a single type material or multiple layers of the same or different material. The sheath may comprise one or more layers of polymeric or other material to provide physical, mechanical, electrical insulating and/or chemical protection for the underlying cable components. Crosslinked polymers such as crosslinked polyethylene (XLPE) are used as electrical insulation layers or jackets for various electrical power cable types such as type XHHW, type RHH/RHW, and type USE cables.
Installation of electrical power cable often requires that it be pulled through tight spaces or small openings in, and in engagement with, narrow conduits, raceways, cabletrays, or passageways in rafters or joists. This becomes problematic since the exterior surface of the cable sheath normally has a multitude of forces acting on it, therefore requiring a large pulling force. Moreover, installation parameters include maximum allowable cable pulling tension and/or sidewall pressure limits. Exceeding these limits can result in degradation of the cable, physical damage and inferior installation.
To overcome this problem, the general industry practice has been to coat the exterior surface of the cable sheath with a lubricant at the job site in order to reduce the coefficient of friction between this surface and the conduit walls or like surfaces, typically using vaselines or lubricants produced specifically for such purpose, such as Yellow 77® (hereinafter, “Y 77”). However, applying a lubricant like Y 77 to the finished cable at the job site poses problems, principally the additional time, expense and manpower required to lubricate the finished cable surface at the job site as well as to clean up after the lubricating process is completed.
Alternative solutions have been proposed, including the application of a separate lubricant layer after the polymeric sheath has been formed or extruded during the manufacturing of the cable, or the application of granules of material to the still-hot sheath during the extrusion process, which granules are designed to become detached when the cable is pulled through the duct. These solutions not only require major alterations of the manufacturing line, but result in a loss in manufacturing time, increased economic costs, and undesirable fluctuations in the geometrical dimensions of the cable sheaths. Other proposed solutions have involved spraying, dipping, or otherwise externally applying a “pulling” lubricant to the exterior surface of the sheath, but these techniques have not been satisfactory for all conditions of service.
As a result, a major breakthrough in this area has been the development of an innovative process by which a preselected pulling lubricant, of appropriate type and sufficiency, is internally introduced during the cable manufacture into the material that is to form the sheath, so that the pulling lubricant, either by migration through, or permeation throughout, the sheath becomes available at the exterior surface of the cable sheath at the time of the cable's installation, and is effective to reduce the amount of force required to install the cable. This process is described in U.S. Pat. No. 7,411,129, assigned to the assignee of the present invention, and is incorporated herein by reference in its entirety. The hereindescribed invention is a specific improvement to such process, as applied to crosslinked insulation of the sheath.
It is important to an understanding of the present invention to know the difference between what are referred to as “pulling lubricants” and what are “processing lubricants.” A pulling lubricant is a lubricant that appears at the outside surface of the sheath of the cable and is effective to reduce the force necessary to install the cable through building conduits and the like. A processing lubricant is lubricating material that is used to facilitate the cable manufacturing process, such as improving the flow of polymer chains during any polymer compounding as well as during the extrusion processes while the polymer is in its molten or melt phase. Cable manufacturers have long used processing lubricants, such as stearic acid or ethylene bis-stearamide wax, as a minor component of the polymeric compound from which the cable sheath is formed. Because a processing lubricant is normally not effective except when the polymer is in this melt phase, the effect of a processing lubricant as an external lubricant is essentially non-existent in the final hardened polymer sheath of the cable. Even where there may be an excessive amount of the processing lubricant, a separate pulling lubricant would still be required to sufficiently reduce the cable sheaths' exterior surface coefficient of friction as well as minimize the pulling force necessary to install the cable.
Accordingly, there has been a long-felt need for an effective method of providing a pulling lubricant at the exterior surface of finished power cables having insulation formed of crosslinked polymeric material, in which the pulling lubricant is effective to reduce the required installation pulling force.